1. Field of the Invention
The present invention relates to an image reading apparatus having a plurality of channels and an image reading method.
2. Related Background Art
Along with the recent progress of a semiconductor process and of production technology, a contact image sensor (CIS) is used in a scanner for consumers because of its characteristics that the cost is inexpensive and it requires only a smaller amount of light from a light source and other characteristics.
A scanner for consumers in recent years has a main scanning period of about 10 to 20 ms while an image reading apparatus, what is called, a copying machine has a main scanning period of about 300 μs, which is two orders of magnitude faster than the scanner.
Here, as one of the problems that arise in the case where the CIS is used in the image reading apparatus with a higher speed by two orders of magnitude, there is variation of a black offset level (which is a signal level for correcting an input signal level to a reference level of a black signal by calculation, and with this signal level, offset of the signal level difference between input signals can be attained) due to a temperature rise of a sensor chip. Specifically, in a continuous original reading mode capable of reading up to approximately 50 sheets with the use of an auto document feeder (ADF), there is conducted only once at the beginning of a job (for example, an image reading operation based on an operator's instruction or the like) shading correction in which the level difference between effective image signals (for example, the level difference between image signals in a line sensor at the time when a uniform density reference plate is read) is offset. This is because copying productivity falls when the shading correction is conducted for each original. Further, for example, in response to the recent increasing request for lower power consumption, there may be a case where power is not supplied to the CIS when scanning is not performed, but power is supplied immediately before scanning is performed, and then, several tens of originals are continuously copied.
In this case, a temperature of a sensor chip or an analog processor rapidly rises from a cool state for about the first minute, and then, rises gradually. As a result, the black offset level varies. Further, in the case where, for example, an Xe lamp is used as an original illumination light source, the lamp is also a heat generating source besides self-heating of the sensor chip or analog processor. Thus, variation is emphasized.
In the prior art described above, the following problem arises. That is, for example, in the case where it takes three minutes from the beginning of reading the first original to the end of reading the fiftieth original in the continuous reading of 50 originals placed on the ADF, the black offset level greatly changes between the time of reading the first original and the time of reading the fiftieth original.
This is more serious in the case of using a so-called multi-chip sensor in which a plurality of sensor chips are arranged. That is, due to the individual difference between the sensor chips, a variation amount of the black offset level is nonuniform among the plurality of chips. Therefore, the reference level of the black signal differs among a plurality of channels, and the difference in luminance level is caused between image regions corresponding to the respective channels, which leads to remarkable deterioration of image quality.